Two reasons.
One, the closer you get to the equator, the easier it is to determine a true north. The further north you go, the more difficult it is to determine how much off of magnetic north you are due to the degree of declination. Furthermore, there is less precision in determining the angle because the distance between the arcs is closer together, while your measuring instruments retain the same resolution.
The second reason is related to the first. Large objects and points are much, MUCH easier to align in more perfect geometry, in better alignment than small objects. If I have two ropes and three posts, I have line segments AB and BC. B is the junction. Let's say BC is off angle from AB by 1 degree.
The two ropes are a meter long each. Can you detect that 1 degree by eye balling? You probably can't.
OTOH, let's say those line segments were each a hundred meters long. Could you detect it by eyeballing? EVERY, SINGLE. TIME. Because at 1 degree off center by 100 meters, there is a significant deviance in where post C is by actual distance.
All over the world, builders who have no idea what geometry is will know how to make right angles out of a length of rope. (I'm going to leave it to you to ponder how without looking up.) The longer the rope, the close to true 90 you can make a structure. Think about it.
As for why pyramid shaped? I'm going to let you think about why anyone who has actually built shit why it's better to have wider base than a top for any free standing structure. (Seriously? You dumb motherfuckers can't look at a sand castle or an hourglass and intuit why a pyramid shape is best? I honestly can't believe in this day and age that we have humans that can read and write and have access to the collective sum totality of human knowledge at their fingertips can't figure this out for themselves.
Here's a hint. How many square/cube fucking mountains have you seen?